The present invention pertains generally to fluid dispensers and related apparatus used to produce foam-in-place packaging cushions and, more particularly, to an improved system for preventing such dispensers from occluding due to build-up and/or hardening of fluid within the dispenser, without the need for a cleaning solvent.
The invention finds particularly utility in the field of foam-in-place packaging, which is a highly useful technique for on-demand protection of packaged objects. In its most basic form, foam-in-place packaging comprises injecting foamable compositions from a dispenser into a container that holds an object to be cushioned. Typically, a plastic film is used as a barrier between the object and the rising (expanding) foam and as a liner between the foam and its container (e.g., a corrugated box). As the foam rises, it expands into the remaining space between the object and the container, thus forming a custom cushion for the object.
A common foamable composition is formed by mixing an isocyanate compound with a hydroxyl-containing material, such as a polyol (i.e., a compound that contains multiple hydroxyl groups), typically in the presence of water and a catalyst. The isocyanate and polyol precursors react to form polyurethane. At the same time, the water reacts with the isocyanate compound to produce carbon dioxide. The carbon dioxide causes the polyurethane to expand into a foamed cellular structure, i.e., a polyurethane foam, which serves to protect the packaged object.
In other types of foam-in-place packaging, an automated device produces flexible containers, e.g., in the form of bags, from flexible, plastic film and dispenses a foamable composition into the containers as the containers are being formed. As the composition expands into a foam within the container, the container is sealed shut and typically dropped into a box or carton holding the object to be cushioned. The rising foam again tends to expand into the available space, but does so inside the container. Because the containers are formed of flexible plastic, they form individual custom foam cushions around the packaged objects. Exemplary devices for automatically producing foam-in-place cushions in this manner are assigned to the assignee of the present invention, and are illustrated, for example, in U.S. Pat. Nos. 4,800,708, 4,854,109, 5,376,219, and 6,003,288, the contents of each of which are incorporated entirely herein by reference. Because such cushions enclose the foamable composition in bags, such packaging is commonly known as “foam-in-bag” packaging.
One difficulty with the foamable compositions used to make polyurethane foam for foam-in-place packaging is that the foam precursors and resultant foam tend to have somewhat adhesive properties. As a result, the foamable composition tends to stick to objects and then harden thereon into foam. This tendency is particularly problematic inside of the dispenser from which the foam precursors are ejected. As is known, the polyol and isocyanate foam precursors must be withheld from mixing with one another until just prior to injection. In the most common type of dispenser, the two foam precursors enter the dispenser, mix with one another in an internal chamber disposed within the dispenser to form a foamable composition, and then the resultant foamable composition exits the dispenser via a discharge outlet. As the dispenser operates over and over again, particularly in automated or successive fashion, foamable composition tends to build up in the internal mixing chamber and around the discharge outlet of the dispenser, harden into foam, and block the proper exiting of further foamable composition. As a result, the mixing chamber and discharge outlet must be frequently cleaned to ensure continued operation of the dispenser.
Conventionally, a solvent capable of dissolving both the foam precursors and the foamable composition prior to its fully cured state is used to clean foam-in-place dispensers. In order to clean the dispenser on an on-going basis without the necessity of frequent removal of the dispenser from the cushion-making device for manual cleaning and/or disassembly, the solvent is generally supplied to the discharge end of the dispenser from a separate source as disclosed, e.g., in U.S. Pat. Nos. 6,929,193 and 6,996,956, the disclosures of which are hereby incorporated herein by reference thereto.
While the use of a solvent in this manner has proven to be an effective cleaning technique, it adds to the operational expense and complexity of foam-in-place packaging systems, and is generally unfavorable from an environmental standpoint. In addition, excess solvent is generally allowed to flow from the dispenser and into next cushion to be formed, thereby avoiding the necessity of separate disposal of such excess solvent. While this works well in many applications, the solvent can leak through incomplete seals or vent holes in the bag or film containing the foam. Such solvent leakage can be detrimental to the surface appearance of certain types of packaged items, such as wood furniture.
Accordingly, a need exists in the art for an improved means for continually and automatically cleaning dispensers used in foam-in-place packaging, which obviates the need for a cleaning solvent.